Clarify implementation milestones and ordered parallel lanes (#25)

* docs: clarify build step 4 scope and remaining work

* docs: organize remaining implementation into parallel lanes

* docs: map every implementation task to ordered lane steps

* docs: make CI ownership and validation assignments explicit

* docs: require guarded attempt settlement in lane contracts

* docs: state lifecycle and merge-queue completion gates

* docs: make baseline verification handoffs explicit

* docs: complete retry-context and CI handoff requirements

Author: mchwang